1. Field of the Invention
This invention relates to light emitting device, particularly to a ballast resistor for stabilizing the current through a light emitting device.
2. Brief Description of Related Art
FIG. 1 shows a prior art light emitting device 10 with no current limiting resistance. If the current flowing the light emitting device is excessive, a resistor 12 should be inserted in series to limit the power consumption within a specified tolerance. In FIG. 1, the limiting resistor 12 is mounted on the same substrate 15 as the light emitting device 10 and is wire bonded to the top electrode 101 of the light emitting device 10 with its bottom electrode 102 resting over the substrate 15 through a thin metal pad over the substrate 15.
Furthermore, due to production variations, there is a distribution of the current through the light emitting device 10. In the middle of the distribution, the majority of the device satisfies the specification. Some devices lying to the left side of the distribution curve have inadequate current to satisfy the specification. Other devices lying on the right side of the distribution curve have excessive current to satisfy the specification. Either inadequate or excessive current reduces the production yield. In production, the light emitting devices must be sorted out to categorize them. Those with excessive currents have shorter life due to burnt-out, and must be sold at lower price or discarded.
In the prior art FIG. 1, a separate ballast resistor 12 is wire-bonded to the light emitting device to ballast the current. However, the separate ballast requires an extra wire bonding step and occupies extra area.